Mariusbez
Participant
Post count: 674

Page 184 of your manual
http://files.canon-europe.com/files/soft33601/manual/EOS500D_EN_Flat.pdf
Custom Function 3
0: Auto
The flash sync speed is set automatically within a range of 1/200 sec. to
30 sec. to suit the scene’s brightness.
1: 1/200-1/60 sec. auto
When flash is used with aperture-priority AE (f), this prevents a slow
flash-sync speed from being set automatically in low-light conditions.
It is effective for preventing subject blur and camera shake. However,
while the subject will be properly exposed with the flash, the background
will come out dark.
2: 1/200 sec. (fixed)
The flash-sync speed is fixed to 1/200 sec. This more effectively
prevents subject blur and camera shake than with setting 1. However,
the background will come out darker than with setting 1.